In recent years, in the United States and Europe, much attention has been paid to prevention of health problems caused by Hg, Pb, F, and other trace components contained in flue gas discharged from, for example, power plants, factories of various types, and automobiles, in addition to NOX, SOX, and the like contained in such flue gas. Thus, governments are now considering imposing restrictions to reduce the discharge amount of such components to a very low level. Specifically, as has been known, mercury discharged from a coal-fired boiler is mostly released into the atmosphere in the form of metallic mercury having a high vapor pressure, then transformed into an organic mercury compound, and generally ingested by fish, shellfish, and the like organisms, to thereby produce harmful effects to human health. As has also been reported, 20% of American infants' neurologically impaired cases are suspected of being caused by mercury, in view that mercury causes grave deficit to the nervous system of growing infants. As described above, since mercury is highly toxic to humans, attempts have been made in various fields to reduce the discharge amount of mercury.
A typical known example of such attempts is a method in which, in the presence of a NOX-removal catalyst or an improved product thereof, NOX contained in exhaust gas is reduced by NH3, while highly volatile metallic mercury (elemental mercury, Hg) is oxidized into a mercury compound (e.g., mercury chloride); and subsequently, the thus-oxidized mercury compound is removed together with combustion ash or gypsum by means of an electric dust collector or desulfurization apparatus provided downstream of the exhaust gas treatment system (Patent Document 1). This method employs a catalyst containing titanium oxide, and an oxide of a metal (e.g., vanadium or tungsten) serving as an active component.
Meanwhile, in the case of NOX removal of exhaust gas discharged from a boiler in which high-S-content coal (e.g., coal produced in the eastern United States) is used as a fuel, high percent SO2 oxidation may cause an increase in amount of acidic ammonium sulfate precipitated in an air preheater provided downstream of the boiler, resulting in an increase in pressure loss, or may cause discharge of SO3 through a smokestack, resulting in smoke or secondary pollution. Therefore, in recent years, demand has arisen particularly for a NOX-removal catalyst which realizes low percent SO2 oxidation. In general, such a NOX-removal catalyst contains a small amount of vanadium, which is an active component of the catalyst.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2005-125211